Motor vehicles – Steering gear – With electric power assist
Reexamination Certificate
2003-10-27
2004-12-21
DePumpo, Daniel G. (Department: 3611)
Motor vehicles
Steering gear
With electric power assist
C074S409000, C074S427000
Reexamination Certificate
active
06832662
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electric motor assist type power steering apparatus. More specifically, the present invention relates to an electric-motor-controlled power steering apparatus, which reduces the steering effort of a driver by applying power (steering torque) generated by an electric motor to a steering system.
2. Prior Art
An electric motor assist type power steering apparatus (hereinafter defined as “power steering apparatus”) supplies an assistant steering force to a steering system in order to assist a driver in turning a steering wheel. Vehicles equipped with power steering apparatus are in widespread use. In such vehicles, since smooth turning of the steering wheel is enabled by the power steering apparatus, the driver can turn the steering wheel with ease.
As an example of these kinds of power steering apparatus, a pinion assist type electric power steering apparatus, in which a worm shaft and a worm wheel are used in order to transmit an assistant steering force generated by an electric motor to a steering shaft, is known. The power steering apparatus, disclosed in the Japanese patent unexamined patent publication H11-43062, is one example thereof.
In this power steering apparatus, as shown in
FIG. 8
, an electric motor
51
, which is used to provide an assistant steering force to a steering shaft, is provided. When the steering wheel is rotated in order to turn the steering shaft, the electric motor
51
supplies the assistant steering force to help rotate the steering shaft. The electric motor
51
is connected to a worm shaft
52
. A worm gear
52
A of the worm shaft
52
is engaged with a worm wheel
54
, which is coaxially arranged with respect to a pinion shaft
53
.
Ends
52
B and
52
C of the worm shaft
52
are supported by ball bearings
55
and
56
. The assistant steering force generated by the electric motor
51
is transferred to the pinion shaft
53
through the worm gear
52
A and the worm wheel
54
, which are engaged with each other. Thereby, the effort that must be exerted by a driver to turn the steering wheel is alleviated by the assistant steering force.
In the power steering apparatus
50
disclosed in the above described. Japanese unexamined patent application, however, ends
52
B and
52
C of the worm shaft
52
are only supported by ball bearings
55
and
56
, with the occurrence of backlash.
Therein, since the flexural rigidity (also referred to as flexural resistance, the capacity of an object to resist flexing when stress is applied to thereto) of the worm shaft
52
is relatively low, the worm shaft
52
tends to be easily deformed by the load applied to the worm shaft
52
. Therefore, when the steering wheel is turned back in the other way, since the steering torque sensed by a torque sensor (not shown) decreases, the electric motor
51
is controlled so as to provide a decreased assistant steering force.
It is desirable to enable the steering wheel to be turned back smoothly, by rotating the electric motor
51
through the worm wheel
54
and worm shaft
52
. But, if the worm shaft
52
has flexed, since the assistant steering force has decreased, the bending moment for affecting the worm shaft
52
decreases. When a restoring force is applied to the worm shaft
52
, restorative deformation of the worm shaft
52
occurs.
Since the restorative deformation disturbs the rotation of the worm gear
52
A, the assistant steering force from the electric motor
51
is not transmitted to the worm wheel
54
. Thus, the proper relationships between the torque sensor, the control unit, and the electric motor
51
are impaired. These relationships are further impaired due to the moment of the inertia of the electric motor
51
. Thus, the operational ability of the steering wheel is becomes worse.
In the electric power steering apparatus
50
, moreover, an excessive gap or clearance may develop between the worm gear
52
A and the worm wheel
54
as the worm shaft
52
flexes. When such clearance arises, the backlash between the worm gear
52
A and the worm wheel
54
can become large. Thus, the assistant steering force generated by the electric motor
51
cannot be transferred completely to the steering shaft.
These disadvantages caused by the distortion of the worm shaft
52
are especially pronounced when the assistant steering force generated by the electric motor
51
is large.
SUMMARY OF THE INVENTION
The present invention relates to an electric power steering apparatus, that transmits a steering force to be added to the steering wheel, in compliance with the steering force exerted by the driver.
This electric power steering apparatus includes an electric motor for generating the assistant steering force, a worm shaft engaged with the electric motor so as to be rotated thereby, a worm gear disposed on the worm shaft and engaged therewith so as to rotate therewith, a worm wheel engaged with the worm gear on the worm shaft so as to be rotated by the worm gear, and a pinion shaft engaged with the worm wheel so as to rotate therewith. The present invention also includes a rack shaft engaged with at least one steered wheel so as to displace the steered wheel in an axial direction of the rack shaft, and first and second supporting units supporting the first and second ends of the worm shaft. The present invention further includes a distortion prevention unit engaged with the worm shaft at the middle thereof.
In the electric power steering apparatus according to the present invention, both ends of the worm shaft are supported by the supporting units without play. Thus, the worm shaft can be supported with high flexural rigidity.
Accordingly, since distortion of the worm shaft is avoided, the steering force generated by the electric motor may be reliably transferred to the pinion shaft. Difficulties with the operational ability of the steering wheel thus may be avoided.
In the electric power steering apparatus, preferably, each of the supporting units is composed of ball bearings for supporting their respective ends of the worm shaft.
According to this electric power steering apparatus, both ends of the worm shaft are supported without play using ball bearings. Thus, the worm shaft can be rotated while being well supported. Because ball bearings are general-purpose parts, this may be accomplished without using specialized parts. Consequently, a cost reduction may be attained.
Alternatively, in the electric power steering apparatus, preferably, the supporting units include ball bearings which support the first end of the worm shaft, and a needle bearing which supports the second end of the worm shaft.
According to this electric power steering apparatus, the first end of the worm shaft that is joined with the electric motor is supported by ball bearings. Since the first end of the worm shaft is joined with the electric motor, flexural rigidity in the first end of the worm shaft is comparatively high. However, if left unsupported, the flexural rigidity of the other end becomes relatively low.
In the present electric power steering apparatus, therefore, the second end of the worm shaft is supported by a needle bearing, without play. When the worm shaft is supported by the needle bearing without play, the flexural rigidity along the worm gear as a whole may be higher. Difficulties with the operational ability of the steering wheel thus may be avoided.
Furthermore, the present invention relates to an electric power steering apparatus including an electric motor generating an assistant steering force to be added to the steering system, a worm shaft engaged with the electric motor so as to be rotated thereby, a worm gear disposed on the worm shaft and engaged with the worm shaft so as to rotate therewith, a worm wheel engaged with the worm gear on the worm shaft so as to be rotated by the worm gear, pinion shaft engaged with the worm wheel so as to rotate therewith, and a rack shaft engaged with at least one steered wheel so as to displace the steered wheel along the axial direction of the
Shimizu Yasuo
Terada Yasuhiro
Watanabe Katsuji
DePumpo Daniel G.
Honda Giken Kogyo Kabushiki Kaisha
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